go: use testenv.Command instead of exec.Command in tests

go: use testenv.Command instead of exec.Command in tests

This may help to diagnose whether the hang observed in
https://build.golang.org/log/d03db1d27515a4f7e91502e8b58bc83f6e2c04be
is related to #56180.

Updates #56180.
Updates #54773.

Change-Id: I81d37e55a35f876905ceabc103fcf0d1ff348e2f
Reviewed-on: https://go-review.googlesource.com/c/go/+/450615
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>